Depression is an illness that is poorly understood and treatments are only effective in a certain percentage of the population. Antidepressant medications, including selective serotonin reuptake inhibitors (SSRIs), are the most prescribed antidepressant medications and are effective in animal models of depression. SSRI's act indirectly via 5HT receptors including receptors that couple to heterotrimeric G proteins. Regulators of G protein signaling (RGS) are a family of intracellular proteins that negatively modulate receptor-mediated G protein signaling. Using mutant mice expressing an RGS-insensitive G?i2 protein we have identified a pathway downstream of 5HT1A receptors that mediates antidepressant and anti-anxiety behaviors. This led us to consider that RGS protein modulation of 5HT1A receptor signaling may play an important role in depression and so present a novel target for the treatment of depression. However, there are >20 mammalian RGS proteins and a general inhibitor will likely have a broad range of effects. In this proposal we seek to understand the function of G?i2-coupled 5HT1A receptors in animal models of depression by identifying the location of receptors responsible for the observed phenotype, the biochemical mechanism underlying the antidepressant-like effect, and the particular RGS proteins involved. The proposed work will use a combination of behavioral, biochemical and molecular biological approaches in genetically modified mice with knock-in of RGS-insensitive G?i2 protein, including conditional knock-in animals that avoid problems associated with compensatory changes and allow for site-specific expression of the G?i2 mutation. These studies will provide knowledge of RGS proteins in serotonin signaling in animal models of depression and identify novel targets for antidepressant therapy. In particular, RGS proteins may be direct drug targets and/or act to promote the antidepressant actions of currently available SSRI's without enhancing their unwanted effects. PUBLIC HEALTH RELEVANCE: Depression is a poorly understood illness that affects approximately 10 million Americans. Moreover, treatments for depression are ineffective in a majority of patients. We have recently identified a transgenic mouse model that shows antidepressant-like behavior and responds very well to selective serotonin reuptake inhibitors (SSRIs). These mice carry a mutation that renders specific intracellular signaling proteins (G proteins) unresponsive to their endogenous regulators (RGS proteins). This mutation also increases the effectiveness of drugs targeting the serotonin 5HT1A receptor, a receptor that is linked to several mood disorders. In this proposal we seek to increase our understanding of 5HT1A receptors using behavioral and biochemical measures and identify the particular RGS protein(s) involved in their regulation. Our aim is to identify novel targets for the development of agents to treat depressive illness.